Loop a video mask in the compositor?
Let's say your setup on the compositor is like this:



Unplug the input from the mix node and make the colors black and white and render the mask as an image sequence.



And import them as an image sequence node:



And use the sequence instead of the mask node.



Make sure the cyclic and auto refresh options are enabled.



The mask will be used in an endless cycle:



The main disadvantage of this solution is that if you need to make changes on the mask's shape you have to re-export/re-load the images.